1. Field of the Invention
The present invention relates generally to the encoding of data for transmission over a serial link and more particularly to the binary-to-ternary encoding and decoding of data transmitted over twisted-pair wire.
2. Art Background
Prior art local area networks (LANs) such as Ethernet.TM. typically utilize serial links for the transmission of binary data between nodes. Typical examples of prior art serial links are coaxial cable and twisted-pair wire. Transmission of digital data over the serial link is accomplished by varying the analog voltage levels of the serial link. One concern of transmitting data over such serial links is maintaining the reliable determination of transmitted data in view of the capacitance of the serial link itself. Because the conductor of the serial link acts as a capacitor, care must be taken to ensure that no charge is stored in the serial link. If the serial link is allowed to store charge, the transmitted data may be incorrectly sensed.
A typical prior art method for ensuring that there is no charge stored in the serial link of the LAN is Manchester encoding. Manchester encoding ensures that in any given packet or frame of transmitted data, the number of logic O's transmitted and the number of logic 1's transmitted are equal. This equality is termed "zero-balanced." Zero-balanced coding schemes ensure that the net charge, or direct current (DC) balance of the serial link is zero.
One limitation of Manchester encoding schemes is that Manchester encoding requires a doubling of the bandwidth for the serial link. When the desired bandwidth of the LAN is very high, e.g. 100 MBp/s, and the serial link is twisted-pair, Manchester encoding cannot be supported. This is because twisted-pair cannot support a bandwidth of 200 MBp/s over a significant distance, e.g. 100 meters. Therefore, Manchester encoding cannot be used in all situations. However, Manchester encoding has attributes that are important to reliably transmit data over serial links. Specifically, it is desirable that the DC balance of the serial link be maintained at zero. Therefore, a new encoding system for transmitting zero DC balanced data over a high bandwidth LAN is required.